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We develop a detailed chemical network relevant to the conditions characteristic of prestellar core collapse. We solve the system of time-dependent differential equations to calculate the equilibrium abundances of molecules and dust grains, with a size distribution given by size-bins for these latter. These abundances are used to compute the different non-ideal magneto-hydrodynamics resistivities (ambipolar, Ohmic and Hall), needed to carry out simulations of protostellar collapse. For the... Topics: Astrophysics, Solar and Stellar Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1604.05613

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Context. Ultraviolet radiation plays a crucial role in molecular clouds. Radiation and matter are tightly coupled and their interplay influences the physical and chemical properties of gas. In particular, modeling the radiation propagation requires calculating column densities, which can be numerically expensive in high-resolution multidimensional simulations. Aims. Developing fast methods for estimating column densities is mandatory if we are interested in the dynamical influence of the... Topics: Instrumentation and Methods for Astrophysics, Astrophysics Source: http://arxiv.org/abs/1410.0624

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Complex turbulent motions are ubiquitously observed in many astrophysical systems. Their origin, however, is still poorly understood. When cosmic structures form, they grow in mass via accretion from the surrounding environment. We propose that this accretion is able to drive internal turbulent motions in a wide range of astrophysical objects and study this process in the case of galaxies, molecular clouds and protoplanetary disks. We use a combination of numerical simulations and analytical... Source: http://arxiv.org/abs/0912.0288v2

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In this paper, we provide a more accurate description of the evolution of the magnetic flux redistribution during prestellar core collapse by including resistive terms in the magnetohydrodynamics (MHD) equations. We focus more particularly on the impact of ambipolar diffusion. We use the adaptive mesh refinement code RAMSES to carry out such calculations. The resistivities required to calculate the ambipolar diffusion terms were computed using a reduced chemical network of charged, neutral and... Topics: Solar and Stellar Astrophysics, Astrophysics Source: http://arxiv.org/abs/1509.05630

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We present an analytical determination of the star formation rate (SFR) in molecular clouds, based on a time-dependent extension of our analytical theory of the stellar initial mass function (IMF). The theory yields SFR's in good agreement with observations, suggesting that turbulence {\it is} the dominant, initial process responsible for star formation. In contrast to previous SFR theories, the present one does not invoke an ad-hoc density threshold for star formation; instead, the SFR {\it... Source: http://arxiv.org/abs/1110.0033v2

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We derive an analytical theory of the prestellar core initial mass function based on an extension of the Press-Schechter statistical formalism. With the same formalism, we also obtain the mass spectrum for the non self-gravitating clumps produced in supersonic flows. The mass spectrum of the self-gravitating cores reproduces very well the observed initial mass function and identifies the different mechanisms responsible for its behaviour. The theory predicts that the shape of the IMF results... Source: http://arxiv.org/abs/0805.0691v1

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The present paper extends our previous theory of the stellar initial mass function (IMF) by including the time-dependence, and by including the impact of magnetic field. The predicted mass spectra are similar to the time independent ones with slightly shallower slopes at large masses and peak locations shifted toward smaller masses by a factor of a few. Assuming that star-forming clumps follow Larson type relations, we obtain core mass functions in good agreement with the observationally... Source: http://arxiv.org/abs/1304.6637v1

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We present a new algorithm, named Autonomous Gaussian Decomposition (AGD), for automatically decomposing spectra into Gaussian components. AGD uses derivative spectroscopy and machine learning to provide optimized guesses for the number of Gaussian components in the data, and also their locations, widths, and amplitudes. We test AGD and find that it produces results comparable to human-derived solutions on 21cm absorption spectra from the 21cm SPectral line Observations of Neutral Gas with the... Topics: Instrumentation and Methods for Astrophysics, Astrophysics of Galaxies, Astrophysics Source: http://arxiv.org/abs/1409.2840

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Abridged: We study the properties of clumps formed in three-dimensional weakly magnetized magneto-hydrodynamic simulations of converging flows in the thermally bistable, warm neutral medium (WNM). We find that: (1) Similarly to the situation in the classical two-phase medium, cold, dense clumps form through dynamically-triggered thermal instability in the compressed layer between the convergent flows, and are often characterised by a sharp density jump at their boundaries though not always. (2)... Source: http://arxiv.org/abs/0808.0986v2

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We report the results of radiation-magneto-hydrodynamics calculations in the context of high mass star formation, using for the first time a self-consistent model for photon emission (i.e. via thermal emission and in radiative shocks) and with the high resolution necessary to resolve properly magnetic braking effects and radiative shocks on scales Source: http://arxiv.org/abs/1110.2955v1

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Cosmic rays are the main agents in controlling the chemical evolution and setting the ambipolar diffusion time of a molecular cloud. We summarise the processes causing the energy degradation of cosmic rays due to their interaction with molecular hydrogen, focusing on the magnetic effects that influence their propagation. Making use of magnetic field configurations generated by numerical simulations, we show that the increase of the field line density in the collapse region results in a... Topics: Astrophysics of Galaxies, Astrophysics Source: http://arxiv.org/abs/1406.2455

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The main signature of the interaction between cosmic rays and molecular clouds is the high ionisation degree. This decreases towards the densest parts of a cloud, where star formation is expected, because of energy losses and magnetic effects. However recent observations hint to high levels of ionisation in protostellar systems, therefore leading to an apparent contradiction that could be explained by the presence of energetic particles accelerated within young protostars. Our modelling... Topics: High Energy Astrophysical Phenomena, Solar and Stellar Astrophysics, Astrophysics Source: http://arxiv.org/abs/1509.06416

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As long as magnetic fields remain frozen into the gas, the magnetic braking prevents the formation of protostellar discs. This condition is subordinate to the ionisation fraction characterising the inmost parts of a collapsing cloud. The ionisation level is established by the number and the energy of the cosmic rays able to reach these regions. Adopting the method developed in our previous studies, we computed how cosmic rays are attenuated as a function of column density and magnetic field... Topics: Solar and Stellar Astrophysics, Astrophysics Source: http://arxiv.org/abs/1503.06817

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Improving our understanding of the earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We discuss recent advances made in this area thanks to detailed mapping observations at infrared and (sub)millimeter wavelengths. Although ambipolar diffusion appears to be too slow to play a direct role in the formation of dense cores, there is nevertheless good evidence that the gravitational collapse of isolated... Source: http://arxiv.org/abs/astro-ph/0312370v1

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We present a simple dimensional argument to illustrate the impact of nonthermal support from turbulent velocity dispersion on the shape of the prestellar core mass function (CMF), precursor of the stellar initial mass function (IMF). The argument demonstrates the need to invoke such support to recover the Salpeter slope in the high-mass part of the CMF/IMF, whereas pure thermal support leads to a much steeper slope. This simple dimensional argument clearly highlights the results obtained in the... Source: http://arxiv.org/abs/1109.2780v1

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In order to study the fragmentation of massive dense cores, which constitute the cluster cradles, we observed with the PdBI in the most extended configuration the continuum at 1.3 mm and the CO(2-1) emission of four massive cores. We detect dust condensations down to ~0.3 Msun and separate millimeter sources down to 0.4" or ~1000 AU, comparable to the sensitivities and separations reached in optical/infrared studies of clusters. The CO(2-1) high angular resolution images reveal... Source: http://arxiv.org/abs/1211.2666v1

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We find exact and explicit solutions of the axisymmetric MHD equations of a self-gravitating polytropic gas. These solutions are able to describe a flat (uniform density) subsonic internal core, contracting homologously, of a collapsing cloud with aspherical pressure, aspherical gravitationnal potential, magnetic field and rotation. Two of the solutions describing the collapse of a rotating and magnetized cloud present outflow at the pole or at the equator. Source: http://arxiv.org/abs/astro-ph/0108467v1

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Molecular cloud structure is regulated by stellar feedback in various forms. Two of the most important feedback processes are UV photoionisation and supernovae from massive stars. However, the precise response of the cloud to these processes, and the interaction between them, remains an open question. In particular, we wish to know under which conditions the cloud can be dispersed by feedback, which in turn can give us hints as to how feedback regulates the star formation inside the cloud. We... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1607.05487

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These lectures attempt to expose the most important ideas, which have been proposed to explain the formation of stars with particular emphasis on the formation of brown dwarfs and low-mass stars. We first describe the important physical processes which trigger the collapse of a self-gravitating piece of fluid and regulate the star formation rate in molecular clouds. Then we review the various theories which have been proposed along the years to explain the origin of the stellar initial mass... Source: http://arxiv.org/abs/1204.5063v1

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Stars are often observed to form in clusters. It is therefore important to understand how such a region of concentrated mass is assembled out of the diffuse medium and its properties eventually prescribe the important physical mechanisms and determine the characteristics of the stellar cluster. We study the formation of a gaseous proto-cluster inside a molecular cloud by performing high resolution MHD simulations and associate its internal properties to those of the parent cloud by varying the... Topics: Astrophysics, Solar and Stellar Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1603.07942

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Most stars are born in the gaseous proto-cluster environment. The knowledge of this intermediate stage gives more accurate constraints on star formation characteristics. We demonstrate that a virialized globally supported structure, in which star formation happens, is formed out of a collapsing molecular cloud, and derive a mapping from the parent cloud parameters to the proto-cluster to predict its properties, with a view to confront analytical calculations with observations and simulations.... Topics: Astrophysics, Solar and Stellar Astrophysics Source: http://arxiv.org/abs/1603.07983

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It is believed that the majority of stars form in clusters. Therefore it is likely that the gas physical conditions that prevail in forming clusters, largely determine the properties of stars that form and in particular the initial mass function. We develop an analytical model to account for the formation of low mass clusters and the formation of stars within clusters. The formation of clusters is determined by an accretion rate, the virial equilibrium and energy as well as thermal balance. For... Source: http://arxiv.org/abs/1207.6776v1

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H2 is the simplest and the most abundant molecule in the ISM, and its formation precedes the formation of other molecules. Understanding the dynamical influence of the environment and the interplay between the thermal processes related to the formation and destruction of H2 and the structure of the cloud is mandatory to understand correctly the observations of H2. We perform high resolution MHD colliding flow simulations with the AMR code RAMSES in which the physics of H2 has been included. We... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1512.05523

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We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6$-$0.4... Source: http://arxiv.org/abs/1005.5653v1

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The formation of protoplanetary discs during the collapse of molecular dense cores is significantly influenced by angular momentum transport, notably by the magnetic torque. In turn, the evolution of the magnetic field is determined by dynamical processes and non-ideal MHD effects such as ambipolar diffusion. Considering simple relations between various timescales characteristic of the magnetized collapse, we derive an expression for the early disc radius, $ r \simeq 18 \, {\rm AU} \,... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1608.02525

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We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate (SFR) and efficiency (SFE), as well as of the mass-to-magnetic-flux ratio (M2FR) in the forming clouds. We find that: 1) Clouds formed by supercritical inflow streams proceed directly to collapse, while clouds... Source: http://arxiv.org/abs/1101.3384v1

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Context. Molecular clouds are known to be turbulent and strongly affected by stellar feedback. Moreover, stellar feedback is believed to drive turbulence at large scales in galaxies. Aims. We study the role played by supernovae in molecular clouds and the influence of the magnetic field on this process. Methods. We performed three-dimensional numerical simulations of supernova explosions, in and near turbulent self-gravitating molecular clouds. In order to study the influence of the magnetic... Topics: Astrophysics of Galaxies, Astrophysics Source: http://arxiv.org/abs/1410.7972

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This paper assesses the roles of the presence of warm H2, and the increased formation rate due to the ion-neutral drift. We performed ideal MHD simulations that include the heating and cooling of the multiphase ISM, and where we treat dynamically the formation of H2. In a post-processing step we compute the abundances of species at chemical equilibrium. We show that CH+ is efficiently formed at the edge of clumps, in regions where the H2 fraction is low, but nevertheless higher than its... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1612.03030

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We review the theories and observations of high-mass star formation emphasizing the differences with those of low-mass star formation. We hereafter describe the progress expected to be achieved with Herschel, thanks notably to Key Programmes dedicated to the earliest phases of high-mass star formation. Source: http://arxiv.org/abs/0806.0703v1

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Outflows and jets are intimately related to the formation of stars, and play an important role in redistributing mass, energy and angular momentum within the dense core and parent cloud. The interplay between magnetic field and rotation is responsible for launching these outflows, whose formation has been generally carried out for idealized systems where the angle $\alpha$ between the rotation axis and large-scale magnetic field is zero. Here we explore, through three-dimensional ideal... Source: http://arxiv.org/abs/1009.0453v2

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We present a new set of analytic models for the expansion of HII regions powered by UV photoionisation from massive stars and compare them to a new suite of radiative magnetohydrodynamic simulations of turbulent, self-gravitating molecular clouds. To perform these simulations we use the Eulerian adaptive mesh magnetohydrodynamics code RAMSES-RT, including radiative transfer of UV photons. Our analytic models successfully predict the global behaviour of the HII region provided the density and... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1507.02981

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The development of parallel supercomputers allows today the detailed study of the collapse and the fragmentation of prestellar cores with increasingly accurate numerical simulations. Thanks to the advances in sub-millimeter observations, a wide range of observed initial conditions enable us to study the different modes of low-mass star formation. The challenge for the simulations is to reproduce the observational results. Two main numerical methods, namely AMR and SPH, are widely used to... Source: http://arxiv.org/abs/0801.3092v1

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Theoretical studies of collapsing clouds have found that even a relatively weak magnetic field (B) may prevent the formation of disks and their fragmentation. However, most previous studies have been limited to cases where B and the rotation axis of the cloud are aligned. We study the transport of angular momentum, and its effects on disk formation, for non-aligned initial configurations and a range magnetic intensities. We perform 3D AMR MHD simulations of magnetically supercritical collapsing... Source: http://arxiv.org/abs/1203.1193v3

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Radiative transfer has a strong impact on the collapse and the fragmentation of prestellar dense cores. We present the radiation-hydrodynamics solver we designed for the RAMSES code. The method is designed for astrophysical purposes, and in particular for protostellar collapse. We present the solver, using the co-moving frame to evaluate the radiative quantities. We use the popular flux limited diffusion approximation, under the grey approximation (one group of photon). The solver is based on... Source: http://arxiv.org/abs/1102.1216v3

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Radiative feedback and magnetic field are understood to have a strong impact on the protostellar collapse. We present high resolution numerical calculations of the collapse of a 1 solar mass dense core in solid body rotation, including both radiative transfer and magnetic field. Using typical parameters for low-mass cores, we study thoroughly the effect of radiative transfer and magnetic field on the first core formation and fragmentation. We show that including the two aforementioned physical... Source: http://arxiv.org/abs/1011.5651v1

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This paper presents follow-up observations of the mid-Infrared dark clouds selected from the ISOGAL inner Galaxy sample. On-the-fly maps of 13CO, C18O and the 1.2 mm continuum emission have been obtained a the IRAM 30-m telescope. A preliminary analysis of the physico-chemical conditions at works in the clouds is given and molecular depletion onto cold grains is discussed. Source: http://arxiv.org/abs/astro-ph/0111458v1

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Understanding the formation of binary and multiple stellar systems largely comes down to studying the circumstances for the fragmentation of a condensing core during the first stages of the collapse. However, the probability of fragmentation and the number of fragments seem to be determined to a large degree by the initial conditions. In this work we study the fate of the linear perturbations of a homogeneous gas sphere both analytically and numerically. In particular, we investigate the... Topics: Astrophysics, Solar and Stellar Astrophysics Source: http://arxiv.org/abs/1411.3177

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What exactly controls star formation in the Galaxy remains controversial. In particular, the role of feedback and magnetic field are still partially understood. We investigate the role played by supernovae feedback and magnetic field onto the star formation and the structure of the Galactic disk. We perform numerical simulations of the turbulent, magnetized, self-gravitating, multi-phase, supernovae regulated ISM within a 1 kpc stratified box. We implemented various schemes for the supernovae.... Topics: Astrophysics of Galaxies, Astrophysics Source: http://arxiv.org/abs/1405.7819

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Stellar clusters form by gravitational collapse of turbulent molecular clouds, with up to several thousand stars per cluster. They are thought to be the birthplace of most stars and therefore play an important role in our understanding of star formation, a fundamental problem in astrophysics. The initial conditions of the molecular cloud establish its dynamical history until the stellar cluster is born. However, the evolution of the cloud's angular momentum during cluster formation is not well... Topics: Astrophysics of Galaxies, Solar and Stellar Astrophysics, Astrophysics Source: http://arxiv.org/abs/1703.05588

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We numerically investigate the dynamics of a 2D non-magnetised protoplanetary disc surrounded by an inflow coming from an external envelope. We find that the accretion shock between the disc and the inflow is unstable, leading to the generation of large-amplitude spiral density waves. These spiral waves propagate over long distances, down to radii at least ten times smaller than the accretion shock radius. We measure spiral-driven outward angular momentum transport with 1e-4 < alpha <... Topics: Solar and Stellar Astrophysics, Astrophysics Source: http://arxiv.org/abs/1509.04859

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Accretion discs are ubiquitous in the universe and it is a crucial issue to understand how angular momentum and mass are being radially transported in these objects. Here, we study the role played by non-linear spiral patterns within hydrodynamical and non self-gravitating accretion disc assuming that external disturbances such as infall onto the disc may trigger them. To do so, we computed self-similar solutions that describe discs in which a spiral wave propagates. Such solutions present both... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1602.01721

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Understanding how accretion proceeds in proto-planetary discs and more generally their dynamics is a crucial issue for explaining the conditions in which planets form. The role that accretion of gas from the surrounding molecular cloud onto the disc may have on its structure needs to be quantified. We perform tri-dimensional simulations using the Cartesian AMR code RAMSES of an accretion disc subject to infalling material. For the aspect ratio of $H/R \simeq 0.15$ and disk mass $M_d \simeq... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1611.07376

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We present a simple statistical analysis of recent numerical simulations exploring the correlation between the core mass function obtained from the fragmentation of a molecular cloud and the stellar mass function which forms from these collapsing cores. Our analysis shows that the distributions of bound cores and sink particles obtained in the simulations are consistent with the sinks being formed predominantly from their parent core mass reservoir, with a statistical dispersion of the order of... Source: http://arxiv.org/abs/1011.1185v1

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Improving our understanding of the initial conditions and earliest stages of protostellar collapse is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. Observationally, there are two complementary approaches to this problem: (1) studying the structure and kinematics of prestellar cores observed prior to protostar formation, and (2) studying the structure of young (e.g. Class 0) accreting protostars observed soon after point mass formation. We... Source: http://arxiv.org/abs/astro-ph/0212492v1

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We present methods and results from "21-cm Spectral Line Observations of Neutral Gas with the EVLA" (21-SPONGE), a large survey for Galactic neutral hydrogen (HI) absorption with the Karl G. Jansky Very Large Array (VLA). With the upgraded capabilities of the VLA, we reach median root-mean-square (RMS) noise in optical depth of $\sigma_{\tau}=9\times 10^{-4}$ per $0.42\rm\,km\,s^{-1}$ channel for the 31 sources presented here. Upon completion, 21-SPONGE will be the largest HI... Topics: Astrophysics of Galaxies, Astrophysics Source: http://arxiv.org/abs/1503.01108

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We study the rotational properties of magnetized and self-gravitating molecular cloud cores formed in 2 very high resolution 3D molecular cloud simulations.The simulations have been performed using the code RAMSES at an effective resolution of 4096^3.One simulation represents a mildly magnetically-supercritical cloud and the other a strongly magnetically-supercritical cloud.A noticeable difference between the 2 simulations is the core formation efficiency (CFE) of the high density cores.In the... Source: http://arxiv.org/abs/1003.5118v2

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The filamentary structure of the molecular interstellar medium and the potential link of this morphology to star formation have been brought into focus recently by high resolution observational surveys. An especially puzzling matter is that local interstellar filaments appear to have the same thickness, independent of their column density. This requires a theoretical understanding of their formation process and the physics that governs their evolution. In this work we explore a scenario in... Topics: Astrophysics, Astrophysics of Galaxies Source: http://arxiv.org/abs/1602.04737

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[Abridged] Theoretical and numerical studies of star formation have shown that magnetic field (B) has a strong influence on both disk formation and fragmentation; even a relatively low B can prevent these processes. However, very few studies investigated the combined effects of B and turbulence. We study the effects of turbulence in magnetized core collapse, focusing on the magnetic diffusion, the orientation of the angular momentum (J) of the protostellar core, and on its consequences on disk... Source: http://arxiv.org/abs/1301.3004v2

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The formation of protostellar discs is severely hampered by magnetic braking, as long as magnetic fields remain frozen in the gas. The latter condition depends on the levels of ionisation that characterise the innermost regions of a collapsing cloud. The chemistry of dense cloud cores and, in particular, the ionisation fraction is largely controlled by cosmic rays. The aim of this paper is to evaluate whether the attenuation of the flux of cosmic rays expected in the regions around a forming... Topics: Astrophysics of Galaxies, Astrophysics Source: http://arxiv.org/abs/1408.5901